Flat panel display devices such as liquid crystal display devices have advantages such as thin-profile, light-weight, and low power consumption. Also, technologies for improving the display performance thereof such as color display, higher resolution, and video capability have been developed. For this reason, the flat panel display devices are currently used in a wide variety of electronic devices including mobile phones, PDAs, DVD players, mobile gaming devices, laptop computers, PC monitors, and TVs.
Against this background, in recent years, development of a liquid crystal display device in which a photo sensor element is provided in each pixel (or one pixel of RGB pixels) in an image display region (display device with built-in photo sensors) has been sought after. In Patent Document 1, for example, a liquid crystal display device in which photo sensor elements made of photodiodes are formed on a pixel region is disclosed. As described above, by incorporating a photo sensor element in each pixel, it is possible to allow a normal liquid crystal display device to have an area sensor function (specifically, a touch panel function or the like). That is, by using the photo sensor elements, which are incorporated in a display device, as an area sensor, a display device having a function of detecting the position of an input pointer such as an optical stylus can be achieved.